Andrew Cornwall
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The automatic tuner for small magnetic loop antennas supported here is the topic of a major article published by TAPR in its on-line magazine Packet Status Register , Winter 2020, No 144: (




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My desire to sell printed circuit boards and connectors on is not possible at this time, because I am travelling outide of Canada, my home, and cannot get through security to post an ad. I have 40 PCB-connector kits with me and will mail them to TAPR members in Canada and USA who request them. Email me at with your call sign and postal address. If I get more than 40 requests, requests from outside of Canada and USA or non-members of TAPR, I will keep your name and email address and notify you when more kits become available and how to obtain them. This may take a couple of months, since more kits will need to be made and connectors ordered. My intended selling price was $18 CDN (~$13.50 US) shipping free. If I do send you a requested kit we can deal with payment later. My primary objective is to encourage interested, technical amateurs to build an autotuner, and to get the word out that this is a very good way to tune small magnetic loop antennas.

Note: a PCB is not necessary for making an autotuner. It just makes it easier. I have wired autotuners using a prototyping board with interconnecting holes, about 4"x5", and used the female end of a 1/8" stereo earphone extension cable to make a pigtail type serial data connector. I've generally used some type of screw-down connector for the 4 stepper motor wires so the top of the case with the electronics is easily removed. Otherwise off-board connecting wires are soldered directly to the board. The schematic diagram in the TAPR article is a pretty good guide for component placement. Wiring style is not critical, except inside the case fairly close connection between the end of the sampling antenna transmission line and reciever diodes is a good idea. After this there is no RF. I recommend sockets for the 18M2+ and 4N35, and headers for the A4988; making these components easily replaceable.


Programming the Picaxe 18M2+ microcontroller requires a Picaxe development program such as the PICAXE Editor 6 for Windows (development programs for other OS are avaiable) downloadable for free from, and an AXE027, PICAXE USB download cable obtainable to purchase from Amazon and many robotics store websites.

The programs may be examined and edited with a Picaxe development program or a text editor.

The stepper motor wires (two coils, two wires per coil) should be attached when running the programs in a powered circuit to avoid damage to the A4988 stepper motor controller module.

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Some Internet servers and computer anti-virus programs object to passing an executable program with a '.bas' extension. Therefore, the two download programs below have name extensions of 'txt' (eventhough the link says '.bas'). To make them useable the extensions should be renamed to 'bas'.

Apparently the HTML version I am using may not support the normal 'download and save' option for saving the two autotuner programs with some browsers. Instead, when the program link is clicked the text of the program is listed in a new tab. In this case to retrieve and save the program: mark the entire text in the tab (ctrl A), copy it (ctrl C), paste it into a text editor, such as Notepad, and use the 'save function of the text editor to save the file with the correct name (including extension '.bas').

file will downlaod with 'txt' extension; rename with 'bas' extension upon saving
'Autotuner Circuit Test' runs through all of the functions of the autotuner circuit: buzzer, LEDs, measuring sample antenna signal* (optional relay off and on), and turning the stepper motor half through sixteenth partial steps. There is also a 10 second time when the motor is on but not stepped for measuring current to the circuit (mostly going tp the stepper motor). As the program runs the results of the program can be seen in the Picaxe development program monitor screen; set the baud rate to 4800. Make certain that a stepper motor is connected to the circuit. A good time to run the test program is before the circuit is mounted in the autotuner box, and circuit fixes are easier.

* Current is applied to the circuit antenna terminals. If too much current is used the receiver diodes and/or optocoupler LED can be destroyed. As a current source I use a 9 volt battery in series with a 2K ohm, or higher, resistance. Changing the resistance alters the current. The article on this page 'Estimating_Full_Transmit_Power.pdf' explains further how to apply current.

file will downlaod with 'txt' extension; rename with 'bas' extension upon saving
'SMLA Autotuner' is the small magnetic loop antenna automatic tuning program. It is explained in detail in the TAPR Packet Status Register , Winter 2020, No 144, article referred to above.

Make certain that a stepper motor is connected to the circuit.

Autotuner_Parts_List_8.pdf 'Autotuner Parts List' is a collection of autotuner construction information - including a parts list. The information is essential for building an autotuner. Read this information before deciding to make an autotuner.
Estimating_Full_Transmit_Power.pdf 'Estimating_Full_Transmit_Power.pdf' explains how to estimate the full transmitting power the autotuner can withstand before endagering the autotuner reciever diodes and/or optocoupler LED. Full transmitting power is not the small power used when autotuning. Rather it is the highest power when transmitting, for example, making a contact. Transmitting power is picked up by the sampling antenna even when not autotuning. In my experience a fairly high voltage variable capacitor would arc before a dangerous level of transmitting power occurs.

Note, 'Estimating_Full_Transmitting_Power.pdf' was written a couple of years ago and pertains specifically to earlier versions of the autotuner program and circuit. The article is still relevant (the 'green' LED referred to is now yellow).

Autotuner_Report_Feb_11_2020.pdf A computer can report data on tuning progress when attached to an autotuner by means of a Picaxe programming cable. (In manual mode another type of report is produced.) During tuning the autotuner sends data as text to the Monitor function of the Picaxe Development Program. "Autotuner_Report_Feb_11_2020.pdf" shows the reports of three sequential autotunings, one in the 20 meter band and two in 40 meters (same frequency but the sensitivity switch is different). These reports are typical of tuning results.

At the start of tuning the stepper motor rotates the variable capacitor one complete turn recording the antenna's highest signal level. The variable capacitor encounters equal capacitances twice during a turn, thus providing the autotuner with two opportunities to discover a maximum signal value. The maximum becomes the goal for the susequent active tuning sweep (i.e. turn).

The report shows step positions and sample signal values for signal levels greater than 10 (10 or lower are not significant); absolute highest signal is about 1,000. There are two stepper motor partial step sizes during the tuning process: half-step when very little signal is measured, and one-sixteenth step as tuning slows and becomes more presice either: in the vicinity of a maximum signal level in the initial turn, or when approaching the tuning goal during the tuning sweep.

In the report all steps are counted as sixteenth partial step equivalents. A sixteenth step turns the variable capacitor 0.05625 deg. At the end of the report there are 10 signal readings over a few seconds to indicate tuning stability. A small 'wiggle' in the tail is not important.

Usually a computer is not connected to an autotuner. Indeed, tuning is automatic whether or not a computer is connected, and an autotuner's LEDs and a buzzer provide tuning status information. A computer,however, adds interesting autotuner diagnostic capability.

Note, at the end of each tuning report I have inserted information about tuning conditions and results. This is not part of the tuning report.

WSPRNet_Experiment_06.pdf This was a fun exercise. In the summer of 2018 I tested an earlier version of an autotuned small magnetic loop antenna using the facilities of and the thousands of WSPR signal monitoring stations worldwide. The article describes the challanges and results of WSPR transmitting in a field at the back of my property, with neither AC power nor Internet availability. The test transmitter was TAPR's $29 module, a QRP TX Shield for WSPR on 20 Meters', controlled by a Raspberry Pi. TAPR published the article in the No. 139, Fall 2018, edition of the Packet Status Register. Subsiquently, the Radio Amateurs of Canada also published the article in the January-February, 2019 edition of TCA, 'The Canadian Amateur'.

The amazing part of the test is the QRP TX Shield transmits a mere 100 mW (one tenth of a watt), which the autotuner was able to tune with a long sampling antenna. The small magnetic loop antenna sent a signal that was received by many WSPR monitoring stations far and wide.

Thank you WSPR monitors and WSPRnet. There is more information about WSPR elsewhere on this website, including how to set up an inexpensive and effective monitoring station.